One such test apparatus is known from the document JP 2002-22770 A. In this case, the known test apparatus has a needle card which is fitted with a gas supply line, with a gas outlet nozzle forcing a gas flow from outside the test area through a gap between a semiconductor wafer and the test housing into the test area, which is open at the top. In this case, the width of the gap and hence the distance between the test housing and the semiconductor wafer are governed by the gas flow in the gap.
This test apparatus has the disadvantage that there is a risk that an unevenly distributed gas flow in the gas gap can lead to tilting and/or touching on one side, and to damage to the semiconductor wafer surface. In addition, there is a risk of contamination adhering to the semiconductor upper face of the semiconductor wafer, or of particles from the environment being carried into the test area by the gas flow which is carried from the outside inwards. The measurement can thus be corrupted by problems in making contact between the semiconductor component structures and the test probes. Furthermore, there is a risk of the test probes being subject to a non-uniform contact pressure.
A further test apparatus is known from the document JP 2001-281267 A, in which a test area which is covered by a transparent plate is surrounded by a test housing. Nitrogen is forced via a gas inlet into the test area, so that the test probes, which are supported by a needle card, are kept free of contamination in the test area.
One disadvantage of this test apparatus is that the distance between the lower face of the test area housing and the upper face of the semiconductor wafer to be tested is sufficiently large that the gas flow does not carry out any supporting function.
A further conventional test apparatus is shown in FIG. 6. In this test apparatus 40 for semiconductor component structures on a semiconductor wafer 6, a needle card 8 holds test probes 7, with the needle card 8 being supported by a gas cushion ring 31. Supporting elements 32 and 33 hold the needle card 8 in an initial position. The supporting elements 32 and 33 are for this purpose held by spring elements 30, which are supported on a test apparatus frame 34. An annular groove 36 on the lower face of the gas cushion ring 31 is supplied with gas pressure via gas inlets 23, so that the needle card 8 can be supported by the gas cushion ring 31 against the spring effect of the spring elements 30, and a gas gap 13 with a gas gap width b can be maintained between the lower face 35 of the gas cushion ring 31 and the upper face 14 of the semiconductor wafer 6 to be tested.
This test apparatus 40 has the disadvantage that it is impossible to preclude the risk of tilting of the gas cushion ring 31, so that uncontrolled touching of the semiconductor wafer 6 can occur. Furthermore, tilting of the gas cushion ring 31 can dangerously reduce the breakdown strength of the gas gap 13. Furthermore, the gas gap width b is dependent on pressure fluctuations in an annular groove 36 of the gas cushion ring 31. In addition, flow inhomogeneities in the gas gap 13 caused by tilting can result in suction effects on particles from the surrounding area, so that the particles can be drawn from the surrounding area into the test area of the test probes 7. Finally, the resonant system formed from spring elements 30 and supporting elements 32 and 33 with the needle card 8 and the gas cushion ring 31 can be caused to oscillate naturally by the outlet-flow speed in the area of the gas gap 13, which results in increased wear to the test probes 7, reducing the life of the test apparatus 40.